Receiver gain control



Feb. 6, 1951- R. CRANE, JR 2,540,935

A RECEIVER GAIN CONTROL Filed spt. 28. 194s s sheets-sheet 1 Feb. 6,1951 R. CRANE, .1R 2,540,935

RECEIVER GAIN CONTROL Snventor -ms vom @05E/a7 @eA/Vf, qe.

F49. .9 E Gttomeg Feb. 6, 1951 R. CRANE, JR

RECEIVER GAIN CONTROL Filed Sept. 28, 1948 5 Sheets-Sheet 5 nvemorm55/e7 CMA/f, Je.

Gttorneg Patented F eb. 6, 1,951

RECEIVER GAIN CONTROL Robert Crane, Jr., Chappaqua, N. Y., assignor toGeneral Precision Laboratory Incorporated, a corporation of New YorkApplication September 28, 1948, Serial N o. 51,552 s claims. (ci.179-171) This invention pertains to the control of gain in radioreceivers. In particular, it pertains to the prevention of front-enddistortion in radio receivers by the use of a special gain controlmethod and means.

This invention has the widest application to all radio receivers inwhich fidelity of signal reproduction or degree of signal distortion areof any importance. This particularly includes all amplitude modulationradio receivers, and f such receivers particularly those for pulsereception. Such receivers include, in the microwave frequency range,radar receivers; and in the low radio frequency range, long rangenavigation system receivers commonly known as Loran receivers. Thisinvention also has application to continuous wave receivers such asthose used for short and long wave broadcast reception.

In Loran systems two transmitted A and "B pulses are received by Loranreceiving equipment, on an air or marine craft from a master and a slavetransmitting station, both located on shore. These pulses appear as pipson a cathode ray tube screen in the receiving equipment, the slavestation B pip bearing a precise time relation to the master station Apip. By manipulation of controls the two pips are adjusted to coincideon the cathode ray tube screen. This involves certain delay features notimportant here but the result is that by such manipulation the exacttime differential between reception of the A and B pulses is measured,and reference to a Loran chart gives the position of the craft on a lineof position corresponding to this time difference. Another similar pairof signals is then received from a different pair of stations and aftersimilar manipulation a second line of position is determined, the pointof intersection of the two lines giving the position of the craftcarrying the receiving equipment.

What is important in the present development Yis that the two pips beundistorted in shape by the receiver, or if distorted be distortedalike, so that precise and accurate matching or coincidence of theforward sides of the two Dips 0n the cathode ray tube screen is madepossible.

The A and B pulses as received may be widely different in strength,depending on the difference in distances to the master and slavetransmitting stations, and the heights of the resulting A and B pips onthe screen will be correspondingly different. In fact, provision ismade, in the equipment used herein as an example, for comparison of Aand B pulses which differ in magnitude by 10,000 to 1, and cover a rangemeasured at the 2 receiving antenna terminals from l microvolt t 2volts.

However, in spite of such disparity in strength, received signals mustbe so amplified before display on the screen that the observed vpipsshall be of the same amplitude or height, hence it is required that thegain of the receiver be capable of wide variation and that'a gain changebe automatically elected between the reception of an A pulse and thesubsequent reception of the following B pulse. That is,of course, usualin Loran receivers, but the differential gain control used heretofore toadjust the gain to accommodate the difference in coming signal levels ina pair of signal pulses has inherently been the source of selectivedistortion, producing distortion in each pulse in accordance with itsstrength, so that the shape of a pip resulting from a strong pulse wasradically diierent from that of a pip produced by a weak pulse. Thisobviously reduces the matching of the leading sides of A and B pips insuch a case to mere guesswork because of their diierentshape andintroduces a serious error in the linal result, the determination of theposition of the ship. This error has hitherto been unavoidable in theoperation of all Loran receivers. This error may, however, be completelyeliminated if the shapes of the A and B pips, after being made alike inheight, are exactly the same so that their forward slopes can besuperposed with precision and made to coincide exactly. This isaccomplished in the present invention. The shapes of the two pins arealike, resulting in a reduction of this error to negligible proportions.

In any radio receiver such' as for broadcast amplitude modulationreceptions with large input signal level, one cause of distortionoriginates in the first tube, whether it be a radio frequency amplifieror a mixer, to which signals are fed from the antenna. Such rst-tubedistortion is minimized only when a small signal is received by thefirst discharge tube encountered by the received signal. Many methodshave been proposed for reducing this distortion but none has been morethan quantitatively successful and none has even attempted the solutionin the straightforward manner of this invention, in which by the normaluse of one manual gain control it automatically is insured that for anystrength of antenna signal the first operative tube receives only a verysmall signal and that this tube operates on a favorable part of itscharacteristic curve, yet destroying none of the sensitivity of thereceiver for weakest received signals. This results in a constant smalldistortion which will be completely negligible in most applications, andsince it is constant it may be completely neutralized by suitablenetworks.

One purpose of this invention is to minimize and make constantregardless of applied signal strength the distortion originating .intheiirst tubes of :a radio-receiver.

Another purpose of this invention is, in a Loran receiver, to make theshapes of A and B,l

pips the same regardless of any disparity in the strengths of the A andB receivedapulses pro-f ducing the pips.

This invention will be more gre'adlly. under'- stood from the followingdetailed description,- considered together with the attached drawings,

adjustmenhand producing alternately from each in which: Y

Figure 1 illustrates schematically the wiring of a Loran receiveremploying the instant invention.

f Figures'2 to 8'il=lustrategraphically the volt'- age-time relations atvarious points in theicircuit ofrFig. 1.

` Figure 9 illustrates-a bias circuiti-'equivalent to that ofFig.. l.

Figure 10=is` a graph illustrating nthe relation between th input andoutput-- voltages produced by strongi and weak signals.

: Figure 11 illustrates-themanner ini which Y fronti-end `vdistortionarises.

Figuresrm'wa'ndf 13'illustrate#modifications of the: bias circuit- Vforapplication to radarl receivers.`

a; In Fig. 1* I ioran A-and-B pulses are received on antennaI=I,'app1ied to :a radio frequency amplierstube vvthrough atrarisformerI'I tuned by condensers18l-and'fapplied 'teva yconverter 'I8'. Thisconverter may'include the usual pentagridtube:andran'zoscillating"circuitiwhich, for example, may operate at3.1251. megacycles, so that' if channeli 12. A* vLoran .sgnals" 'of 1.95mc. fre'- quency are. received; :signals of anintermediate frequency offl-.f175-'mcowili be produced and fed to first-intermediatefsfrequencywa'mpliiier tube I9, then to second intermediate'frequency am'- plier tube 2 If. 'The output of this tube` is trans#mittednto adetectordiode-212;where the signal isdemodulatedandstransmittedzthrouglr a video amplifier 2-3to-zthe Lorantimer lfor:utilizatior'i in accordance k'with ILioran practice.

. Gain-control thisLOrau-receiveris accom'- plished by=thefvariationfofethenegative' -bias ap'- plied to tubes I6, il9.f andZlgzbut rthat ofztube IS is.:varied in aumanner radically different fromthat of tubes I9 and 2I and this dilference constitutes-.the essenceroffthisninventionr Gain control .is .automatic-inthe .sense Vvthataa timeda-lter-nation.. of gainfsbetween. twovalues foccurs' in synchronism with.-the frequency .of f alternation 0f., A` -and..B,-.pulses,. .but thegain. 'control is yalso manually lvariable-Y-.in two ways, first, `withrespect to theratio ofApulse gain to-.B `pulse gain, and secondly, with.respect -tothe amount vof gain duringthe ,receptionroftheweaker :oithetwo tYDCS of. pulse.

'Gain control in lan amplitude .modulated broadcast receiver wouldof,course,omit .the vdifferential gain control4 and would have van auto--matic gain control. The) application .of this finvention .to suchareceiver, however,` would otherof .thecathode followers II and I3 arectangular .positive pulse or half-cycle at a frequency of 331/3, 25 or20 cycles per second, succeeded by a half period of like length of lesspositive voltage .'cutput; Components may be so designed that thevoltage at the cathode remains at +250 volts for one-half cycle, thenremains at volts for the next half-cycle; This is illustrated for ltheAcathode follower tubeA II during one cycle in Fig; 2,whereinpotential-'level 91 exists at the cathode of tube II fromthebeginning of the -cycle attime S18-to the half-cycle point at time99, and lower potential level II) I exists from that time-'tothe end ofthe cycle at time IGZ. Fig. 3 represents the simultaneous production ofreversed voltagesat -the cathode of tube l2, level -IElI :existingbetween Vtimes V98 and 99, and level 9'? between times 99 and |02.

" :'Connectionare made from the twocathodes, and 2l', to -the two endsof ra high resistance potentiometer' 28. This potentiometer thereforeduring-alternate half cycles, has its upper end atraifpotential Voft250`volts, with a constant drop throughout its-'length to +100 volts atthe lower end; andduring the remaining half-cycle has a potentialv of+250 Vvolts at the lower end and +100 :volts at the upper end. Slider29, if inzthe center.; will therefore be at a constant potential of +175volts, and in any other position will carry-potential .varyingsynchronously with the counter stage :output variation between twovalues. above' andy below +175 fvoltssby not more than' volts. Figv 4:'shows bylca solid line m3 the-cyclicalfvariation'of potential on thelower endxfof this differential potentiometer, with the variationxfonrthe upper; end shown by a dashed line H14.

f'Condenser.(4I-IV translatesithis direct current alternation l:to analternati-ng current ,removing the 17in- Volt directfcurrent-componentand leaving a'rsquare wavealternatingcurrent with an amplitude `:whichmay be'as great as l'75 volts, the amplitude and phase sensedepending'on the positionvof slider-29; This alternating voltage is.i-llustratedybyilig. -4 when the -{--175volt line is-.foon-sidered to,be the: zeroA line.

Diode .32 clips `this voltage `so that only the negative half-cyclesremain. Its lplate 33 is connectedthrough.resistor 3@ to the output sideof condenser'l, and its cathode 3B is grounded. Positiverhalfecycles-therefore pass through this diode toground, 'but negativehalf-cycles charge grid 3l of triode SSnegatively.

Tube .''lis animpedance-transformer. It has ahigh input impedance togroundfor the negative signals Vsuch asl described above and inconjunction `with/resistor 3d `presents such a high impedance tothefoutput'oi the last counter stage as-lio .im-pose practically 'nodrain whatever upon it. .'Ifhe plate 39"-is' connected to a regulatedsource-or` about plus S5-volts at terminal llI.

Y. Resistor Ilestablishes .the direct current bias 012:'diode 32 atapproximately zero volts, neuu Atralizing the contact or diode potentialthereof.

YCathode 46 of triode 38 is connected through low resistance resistor diand high resistance gain control potentiometer 48 to a source ofnegative potential, which may be -105 volts, at terminal 453. CondenserSi, as well as condensers 52, 53, 54 and Ei are for the purpose ofltering out, bypassing and draining off radio-frequency currents and'toprotect against switching transients, while blocking the paths of directcurrents and of 25-cycle alternating currents. A slider 5l connects anydesired point of the potentiometer 48 through lead 58 with the gridreturn leads of tubes I6, IS and 2l, but this is done through a resist;ance network consisting principally of resistors 6i, i3 and 'M so 'thatapproximately twothirteenths of the voltage of slider 5'! is applied togrids S4 and 66I of intermediate frequency tubes i9 and 2l respectively,While all of the voltage is effective at grid 6? of radio frequency tubeI3.

The voltage of the grid 3l of tube 3S ranges from zero to minus '77volts. This is depicted at its maximum negative value of '77 volts inFig. 5 between times 99 and W2, being the case where slider 2Q is at thelower end of potentiometer 28. The cathode G6 will nearly follow thesevalues and will vary from +2 to -65 Volts under the assumed conditions,as illustrated in Fig. 6. The values of resistors Gl and t8 are soselected ,that this will result in a variation of potential at terminal$3 at the top of potentiometer e8 `from Zero to -66 volts as shown inFig; 7. This gure illustrates the cyclic variations in lead 58 whenslider 5l is at the top of potentiometer 48.

When the slider Eil is adjusted at the bottom of potentiometer G8 novariation in potential is present and a continuous steady voltage of-105 is impressed on grid 6l of tube l5 as indicated in Fig. 8.

Fig. 9 illustrates in a more simplified form the grid bias circuitincluding the potentiometer iS and the bias Supplies for tubes IE, ISand 2l. Resistances 59, S2, $3, E8, ll and 'l2 of Fig. l, althoughnecessary as lters and to prevent zero bias operation, are low inresistance, do not greatly aiect the gain control action andhence areomitted for the sake of simplicity. Resistor 76, Fig. 9, represents theparallelled resistances of equal grid leal; resistors 'i3 and 1li, whichmay be approximately 18,090 ohms. Resistor 6l of approximatelyl.,000,000 ohms and resistor '18 then have the effective voltage ofthepotentiometer 5.8 impressed across them in series, the high-voltageterminal being connected to the grid 67 of tube i5 and the2/is-voltage.terminal 'l5 between resistors Si and 'i6 being connectedto the grids Si and it of tubes i9 and 2| inparallel.

When the slider 57 is at the upper end of potentiometer GB, it is at itsleast negative potential and the receiver is therefore most sensitive.Conversely, when the slider is at the lower end the slider E? is at itsmost negative potential and the receiver is least sensitive. The cutoffbias for tube i6 is about -18 volts so that even the strongest or 2-voltantenna input signals, resulting in a 70 volt grid input signwal willnot operate tube iii at all when its grid is negative by at least 88volts assuming a gain for the input net including transformer il of 35times. This will be the case in both halves of the cycle when slider 51is near the bottom of potentiometer 48, and will be the case even whenthe slider is near the top if the dierential slider 29, Fig. 1, is neareither end of potentiometer 28. .In suchV a case the antenna energy willpass tube I 6 only by Way of the capacities involved, mostly distributedWiring capacity, and will not be amplified but on the contrary will beattenuated. This capacitance serves as a loose coupling between theantenna network !'l and converter it, and does not distort the radio orpulse frequencies at all. The sensitivity of the receiver is such thatthe potentiometers 23 and it are so set as to permit the bias of tube i6to rise closer to zero than1 -13 volts during at least one half-cycleonly when the input signal is exceedingly small, so that the excursionof the input along the tube grid-plate characteristics curve is verysmall and the wave shape of the tube output energy is very like that atthe tube grid. Therefore the wave shape of the output voltage of thereceiver is independent of the magnitude of the input signal.

As an example, let it be assumed that two Loran pulses, A and B arebeing received and that the strength at the antenna terminal Ti, Fig. 1,of pulse A is one volt, while that at the same point of pulse B is oneten-thousandth as strong, or microvolts. The potentiometer slider 23must then be adjusted near one end, so that during the half-cycle thatthe strong pulse A is being received, tube le has a high negative gridbias of, say, -86 volts, but during the half-cycle that weak pulse B isbeing received, it has ra low negative bias, of say, -12 volts. Sincethe cutoff point is -18 volts, the tube will not conduct any part of the1.95 megacycle waves constituting pulse A, but will both conduct andstrongly amplify all parts of the weak waves constituting pulse B. It isnecessary to adjust this degree of amplification of the B pulses byadjustment of slider 51 so that the amplitude of the electronicallyampliiied B pulses leaving I. F. tube 2i will equal the amplitude of theelectrostatically transmitted A pulses leaving that tube. Both pulseswill be distorted elsewhere in the receiver but not materially by thetube I5, as is shown by the curves of input as against output derivedunder test conditions and illustrated in Fig. 10. rhe departure of thesecurves from linearity indicates the degree of distortion which pulsesmay be expected to suffer in passing through the receiver, but the closeresemblance in shape between the curve for weak signals and that forstrong signals establishes graphically that all of the signals passingthrough the receiver are distorted inshape in esesntially the samemanner regardless of their strength. On the cathode ray tube screen,therefore, the resulting A and B pips will appear to be identical inshape and thus may be easily and accuratelyY matched.

The manner in which incoming signals may be distorted in receivers notequipped with the bias system of this invention is depicted in Fig. ll.wherein the curve i8 represents the grid voltage plate current relationof the radio frequency tube. If the tube should be biased at about 2lvolts and an input pulse with peak voltage of some 7 volts be applied asat curve le, the output ramasse .s'idered `nearlyf=af1=straighti:lineover theamplitude `oilthesignal:excursion, sothat the tube conducts4'during all parts of the radio frequency cycle :and pulse? frequencycycle, and so that the output pulse shape'is like the input shape.

`1The :manner in which the vbias gain control "system of this inventionis applied to pulsed microwave receivers of various types is shown iin.Figs. 12 and 13. In Fig. 12 the invention is illustrated as ap-plied ina narrow band radar receiver to `the first three discharge tube stages`83, y8d and 85 ofthe intermediate frequency am- 'plien the stage`nearest the antenna, stage 83, Ybeing biased by the full negative biasvoltage and the next two stages -34 and-St, being biased byapproximately 2/13 of thebias voltage. This bias voltage adjustedmanually by slider 8l on the potentiometer dit, or by any equivalentautomatic 'gain vcontrol system, is applied directly to ythe grid returnof stage S3, and a voltage reduced by a voltage dividing networkconsisting of resistors I89 and Si is applied to the grid returns ofstages 81% and 556. -Such a circuit is applicable 'for receptionoi verystrong signals.

For use in a wide band radar receiver a slightly .modified circuit asshown in Fig. 13 is preferred, Where the rst tube -96 lis never biasedbeyond cutoff and gain control vof the invention is applied to thesecond tube 'S2 relative to the thirdland fourth tubes 93 and Sii. Thisis-satisfactory for use in `reception of weak or medium signals, suchsignals in ordinary microwave radar reception being of exceedingly smallamplitude so thatin the absence of this invention any distortion wouldo-ccur in the second stage and not in the iirst. Therefore, conventionalbias may be applied to 'the rst stage, and is preferred to improve-thesignal-noise ratio.

n all of the descriptions given of the application ofthe instantinvention the method of gain controlfconsisting of the application ofnegative voltage to the grid return leads has been employed, andtthismethod is deemed to be preferable. However, 'other Ways of controllingtube bias may'be used with the employment of the general method of thisinvention and the circuits may readily be devised by those skilled inthe art. One such alternative method may for example, consist of thevariation o positive potentials applied to tube cathodes while keepingthe grid return potentials Xed.

What is claimed is:

vl. A gain control for a radio receiver having a plurality of amplifyingstages in cascade comprising a source of variable bias potential, a'voltage divider network connected thereacross, a circuit connecting oneterminal of said divider 'network to a -gain control electrode of one ofsaid amplier stages for impressing the voltage developed across saiddivider network on said electrede, a circuit connecting an intermediateterminal of-said divider network to the gain control velectrode of atleast one succeeding stage whereby the vpotential impressed onsaid-succeeding stage a fraction of that impressed on said rst menticnedstage, said variable biasing potential having as a maximum a potentialsufficient to operate 'said rst stage well below cutoi so that even"higlr-intensity signals are transmitted therethrough only by thecapacity coupling of said stage and the divider network being soproportioned thatthe fractional, potential impressed on thesmzceedingstage is never below the cuton bias sof'tsaid succeedingstage.

:2. .fAL gain control in-accordance with .claim 1in-whichsaid'lgainpontrol electrodes constitute '-thefcontrlelectrodes:of said amplifier stages.

idrop thereacross, :manual means for varying the voltage-supplied vtosaid divider network by said 'voltage sourcevover la range of voltagewhose maximum issulclent tofoperate said first ampliiiertstagewell-belowcutoff so that even high intensity signals `:are `transmittedtherethrough 1only11by the capacity coupling of said stage and *saidintermediate terminal being located at such -a `point thatlthe :lesservoltage impressedon said succeeding stages is never below the cutoffbias thereof.

:4. Afga'inicontr'ol for la radio receiver adapted to `alternatelyreceive "time related pulse signals ofidifferentsignalfstrengths, aplurality of amplier-stag'es therefon'la rst source of -biaspotentiaLlmeans for cyclically varying said biaspotential'initimed'lrelation to'the reception of ksaid.alternatepulsesignals, manual means for varying v.thelamplitude i of:said cyclical Variation, a secondsourceof bias potential of fixed value,a potentiometer.connected'between said rst and vsecondbiaspot'entialsources, a voltage divider networkfic'onnecte'd betweenthe movable contactlofsaid'potentiometer and the common terminalofi-,said potentiometer and said first source of bias potentiaLracircuit connecting one terminal ofsaid voltage dividerV to the gaincontrol electrode of a:.first amplifier stage, a circuit connecting anintermediate terminal of said voltage divider.networktolthe gain controlelectrodes )ofa' plurality of amplifier stages succeeding said rstamplier'lstagersaid voltage divider being so proportionedand the rangeof adjustment of said first vbias potential'andsaid potentiometer beingsuch'that-when adjusted Vfor the reception of all :butvery'weak signalssaid rst amplier stage soperated belowl cutoff while said succeedingstages @are ifmainta'ined in their operating range.

'5. A gainfcon'trol'for afradio receiver having aplurality"ofiamplier.stages in cascade comprising, amanually operablekbias control for varying Vthe bias potentialapplied tosaid stagesincluding a potential divider'connected to said manual- 'ly'operableIbiaslcontrolrand the gain control electrodesof saidfsta'ges'for'applying a proportionally lesser bias potential toi-a plurality ofsucceeding 'amplierstages than toapreceding stage, said 4manualbiasfcontrollbeingoperable overa range su'iiciently widesothat at itsmaximum the bias potential applied tosaid preceding stage cause ittooperatewellbelow cutoff and signals are transmittedltherethrough solelyby the capacitive admittance of 'said stage and said potential dividerbeing so. proportione'dthat the lesser bias potential impressed on saidsucceeding amplifier stages isnever below the cutoff potential thereof.regard-less vof the variation of said bias control.

6. Argainvcontrol.according to claim 5 in which rsaidprecedingstageisaradio frequency ampli- Iier stagefand-saidsucceedingistages are interme-'diate frequency amplifier stages.

.LNgainzcontrol accordinglto claim 5 in which said preceding andsucceeding stages are intermediate frequency amplier stages.

8. A gain control for a radio receiver adapted to alternately receivetime related pulse signals of different signal strengths, a plurality ofamplier stages therefor, a rst source of bias potential, means forcyclically varying said bias potential in timed relation to thereception of said alternate pulse signals, manual means for varying theamplitude and sense of said cyclical variation, a second source of biaspotential of Xed value, a thermionic tube having at least an anode,cathode and control electrode, a circuit impressing said rst source ofbias potential on said control electrode, a potentiometer connectedbetween said cathode and said second source of bias potential, a voltagedivider network connected between the movable contact of saidpotentiometer and a terminal of xed potential, a circuit connecting oneterminal of said voltage 20 divider network to the gain control.electrode of one amplier stage, a circuit connecting an intermediateterminal of said voltage divider netapplied to said succeeding amplifierstages is never below the cutoff bias thereof.

ROBERT CRANE. J n.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 2,129,028 Roberts Sept. 6, 19382,459,798 Dettman Jan. 25, 1949

